Medulloblastoma (MB) is the most common malignant brain tumor in children. We were the first to report on a growth-suppressive effect of PID1 (Phosphotyrosine Interaction Domain containing 1) in MB, constituting the first reported link of PID1 to MB and to cancer overall. We showed that PID1 slowed growth and induced apop- tosis of cell lines from three types of brain tumors, MB, glioma, ATRT, and that lower tumor PID1 mRNA level in MB and glioma tumors correlated with shorter patient survival. Recently we showed that PID1 sensitizes cell lines of MBs and gliomas to chemotherapy, suggesting a potential therapeutic value. Interestingly, PID1 was required for the anti-MB effect of cisplatin, a mainstay of MB chemotherapy. Although cisplatin increased PID1 mRNA, it decreased PID1 protein level in a manner restored by proteasome inhibitors, suggesting that cisplatin promoted proteasomal degradation of PID1. We therefore hypothesize that stabilization of PID1 protein will enhance the anti-MB effect of cisplatin. This hypothesis will be examined in two Specific Aims: 1) To determine PID1 amino acids that are post-translationally modified to mediate cisplatin-dependent deg- radation of PID1 and test if mutating these amino acids to stabilize PID1 will augment the anti-MB effect of cisplatin in culture and in vivo. 2) To determine if proteasome inhibitors augment MB response to cisplatin, if this is via PID1 stabilization, and if the combination is safe and effective against MB in vivo in brains of BarTeL mice. This project will be in cultured MB cells and in our innovative transgenic mouse model for MB, BarTeL and will also test a new brain permeable proteasome inhibitor, marizomib, against intracranial MB in mice. At its con- clusion we will have defined the cisplatin-induced post-translational modifications of PID1, focusing on those that affect protein stability of PID1 and ways to stabilize PID1 in order to enhance efficacy of cisplatin. The potential impact of the knowledge gained is that it will support future translation for patient benefit. In the short term our findings may lead to pre-clinical testing of combination of the new brain-permeable proteasome inhibitor in combination with cisplatin in patients with recurrent MB. In the long term, the knowledge gained will inform our planned design of PID1-based therapeutics to exploit its tumor growth-suppressive effect in MB and other brain tumors.